In previous XENON100 analyses [1]}, [2]}, the recoil energy has been determined using the size of the S1 signal and the relative scintillation efficiency for the nuclear recoils, \(\mathcal {L}_{\textrm {eff}}\) , relative to the 122 keV calibration \(\gamma \)  line of \({}^{57}\) Co  [1]}. WIMPs with a mass below 10 GeV/\(c^2\)  create NRs only up to a few keV, resulting in an S2 signal lower than a few hundred photoelectrons (PE) and an S1 signal that is often not detectable. Therefore for this analysis we only use the S2 signal to infer the energy.
